Conventional comb filters have been used to separate composite video signals into respective luminance (hereinafter “luma”) and chrominance (hereinafter “chroma”) signal components. Typically, comb filters use the inherent characteristics of at least one video signal standard such as NTSC (National Television Standards Committee) or PAL (Phase Alternate Line) to accomplish this separation. In the NTSC standard, chroma is modulated on a subcarrier at approximately 3.579545 Megahertz (MHz). This frequency was chosen because it creates 455 half cycles of subcarrier every line. Therefore, the subcarrier is inverted on every other video line. A two-line comb filter adds two adjacent lines to cancel the chroma, resulting in extraction of the luma and/or subtracts two adjacent lines to cancel the luma, resulting in extraction of the chroma. This processing causes a half line of delay in the chroma signal and high frequency luma. A three-line comb filter adds the line above and below the center line, divides by two, and then subtracts the result from the center line to extract chroma or adds the result to the center line to cancel chroma and to extract luma. A common alternative is to subtract the combed chroma from the composite signal to provide luma.
This process works as long as the lines have the same chroma and high frequency luma values. Adding adjacent lines that are not equal in chroma hue and amplitude does not cancel the subcarrier completely which produces an artifact commonly known as “hanging dots.” The most common fix for this artifact is to switch or “adapt” from comb filter mode back to a band-pass filter to remove the subcarrier and most of the offending sidebands. However, adapting to a band-split filter to separate luma and chroma causes several other problems. These problems include reduced luma resolution and “cross color artifacts” caused by passing high frequency luminance into the chroma path. There are other difficulties in separating the luma and chroma signal components from a composite NTSC and/or PAL video signal. For instance, conventional comb filters have residual artifacts that reduce picture quality and make the resultant video more difficult to compress and to transmit.
PAL is somewhat different than NTSC since it has a 90 degree shift per line requiring 2 lines of delay instead of one to achieve a 180 degree inversion of the subcarrier. Comb filter rules still apply to signals in the PAL standard, and the primary artifacts are still resolution loss, cross-color and hanging dots.
Recent improvements in comb filters are designed to reduce artifacts. Improvements such as those disclosed in U.S. Pat. No. 6,278,495 B1 to Lowe, et al., incorporated herein by reference, relate to multiple directions to comb to reduce artifacts. Further, U.S. Pat. No. 6,462,790 B1 to Lowe, et al., incorporated herein by reference, relates to another improvement in the reduction of artifacts by dynamically controlling the adaptation threshold. However, these improvements do not address cross color artifacts passed in adaptation to band split mode.
Typically, conventional comb filters that are designed for broadcast and video production applications have a wide frequency response that is greater than 4.2 MHz for luma and 1.2 MHz for chroma to take advantage of all of the luma and chroma resolution present in the input signal. Lab experiments with a wide variety of video signals have shown wide chroma bandwidth filters provide relatively better chroma resolutions and a visually sharper picture in most video scenes, however, some of the experiments revealed a conflict in bandwidth requirements. Some picture material causes cross color artifacts in some video scenes that cannot be eliminated by a comb filter but can be eliminated by reducing the chroma bandwidth. Consumer televisions typically do not normally have flat bandwidth filters that are used for broadcast applications. Instead, they typically use inexpensive filters that have less bandwidth. These types of filters do not provide the same resolution performance as flat filters but reduce some of the cross color artifacts as a byproduct of a lower pass band.
Therefore, a need exists for systems and methods for improving video comb filtering with dynamic control of the chroma bandwidth of the video signal.